The present invention relates to a system for detecting the presence of metal in crop material being fed to a forage harvester processing unit and, more particularly, to improved detection apparatus of the type having a magnetic field through which the crop material passes and pickup coils for sensing the presence of metal in the field.
It is not uncommon for objects, such as stones and pieces of wood, plastic or metal, to lie in the path of harvesting machines which cut or pick-up crop material. These objects can cause serious damage to the crop processing means within the machines. Harvesting machines of particular concern are forage harvesters which include crop processing means comprising a rotating cutterhead having a plurality of knives mounted for cooperative relationship with a stationary shearbar.
These high precision crop processing mechanisms require close clearances between the stationary and rotating cutting elements and are usually disposed within the body of the machine whereby access for adjustments, repair and general maintenance is somewhat difficult. The cutterhead normally rotates at a high rate of speed giving rise to the possibility of severe damage caused by foreign objects that are permitted to reach the cutting area. Pieces of wood are not generally a serious hazard nor are stones which generally splinter causing only minor damage to the cutting elements. Plastics and non-ferrous metals are rare, and in any event, relatively soft and hence not hazardous. Thus, the main risk arises from relatively hard ferrous metal objects.
Metallic objects may be whole or broken components that have become detached from machines, tools lost or left lying in a field, discarded hitch pins, fence stakes, scrap dumped by intruders, etc. Additionally, even metal objects of a relatively small size that pass through processing units without causing damage to the cutting means can ultimately poison livestock and, therefore, also present a risk. Thus metallic objects in general, regardless of size, are undesirable and, if possible, should be detected and removed from crop material being fed to the processing unit of a forage harvester to avoid damage to the machine or ultimate loss of livestock that consume the crop material.
Efforts have been made to overcome problems of this nature and the solutions have resulted in devices that have met with varying success. Of the systems known to Applicants, the most successful is the forage harvester metal detector apparatus shown in U.S. Pat. No. 3,959,953 issued to W. L. Garrott, June 1, 1976, and assigned to the assignee of the present invention. Garrott provided for metal detection in the vicinity of the feed rolls and, more particularly contemplated metal detector apparatus mounted within the lower front feed roll of a forage harvester.
Over the years, efforts have been made to continually improve various aspects of commercialized forage harvester metal detectors. Of particular concern has been the desire to provide a system exhibiting reliable detection characteristics. Exemplary of these efforts is the crossed detector coil configuration shown in U.S. Pat. Nos. 3,889,249 to Bennett et al, issued June 10, 1975, and also assigned to the assignee of the present invention. This arrangement has provided for self cancellaton of spurious background signals and thereby improved effectiveness of the system by reducing the number of false detections.
Also of concern during efforts to improve commercialized metal detector apparatus on forage harvesters has been the desire to reduce the likelihood of metal objects passing through the detection area without being sensed. To this end, reference is made to U.S. Pat. No. 4,344,074, Strosser et al issued on Aug. 10, 1982, and also assigned to the assignee of the present invention, which contemplates a system that reduces the possibility of metal passing undetected through the magnetic field.
Thus, as illustrated by the exemplary prior art the need in commercialized forage harvester metal detector apparatus for both avoiding false detections and reducing the number of undetected targets has been recognized. Present day apparatus has enjoyed continuing success but the need for improvements in the reliability and sensitivity to further optimize performance is, of course, always undesirable.